text: Due to complex behavior of fluids around some of the structures such as orifices, gates, and weirs etc., some assumptions are made for the theoretical analysis of the stage-discharge relationship. For example, in case of gates, the pressure at the gate opening is non-hydrostatic which is difficult to model; however, it is known that the pressure at the gate is very small. Therefore, engineers assume that the pressure is zero at the gate opening and following equation is obtained for discharge:
  where:
  : Q, discharge
  : , area of flow
  : g, acceleration due to gravity
  : , head just upstream of the gate
  However, the pressure is not actually zero at the gate; therefore, discharge coefficient, C is used as follows: